The biggest reason to select Paul Ryan is that he's adored by the Republican base. That was one of the main arguments for selecting Sarah Palin, and it made sense in a year when Democrats were "fired up and ready to go". Now, it doesn't. It is true that Mitt Romney is not Mr Conservative, but Republicans are already with him and ready to vote.

• Mitt Romney has trailed in most Gallup surveys, but Republicans are consistently registering as more likely to vote.

Perhaps the polling is missing something; perhaps Republicans really are depressed. There are some indications they may be in Ohio, but these "signs" are mostly speculative and scattered.

Romney's big issue is not that Republicans won't vote for him but that he's losing among independents in the polls that show him down. A larger Republican turnout could also help compensate for losses among independents, but there's a point where the well is going to run dry.

2. Paul Ryan brings more to a Romney campaign running chiefly on the economy

Paul Ryan isn't especially popular in Wisconsin, so the evidence suggests he certainly won't exceed the normal vice-presidential home state effect and might come in a shade below. Two or three percentage points is not enough to put Wisconsin in Romney's column.

There was one poll that indicated Ryan might close the gap for Romney, but I read that result with great caution. Let's recall that polls showed, for instance, John Edwards helping John Kerry to "win" North Carolina in 2004.

Beyond the relative weakness of the reasons to pick Ryan, there are also plenty of substantive reasons not to choose him.

1. Paul Ryan is basically an unknown

Ryan has spent time in the House of Representatives, and has achieved some profile as chairman of the House budget committee, but he hasn't run a competitive race in 14 years. He's never run for an election statewide.

So yes, he's been on TV some, but who really knows what's behind the curtain? I certainly don't. Ryan would, of course, be closely vetted before being picked, but even if there are no outright skeletons in the cupboard, there are bound to be some "hard to explain" embarrassments. Just look at how much stuff has come out about Romney – his taxes, his record at Bain, his Swiss bank account – in the past two months, and he is a tried and tested presidential campaigner.

2. Paul Ryan's Medicare reform plan becomes a central electoral issue

With Ryan now on the GOP ticket, Democrats will seek to make the Ryan plan's proposed cuts to Medicare, aka "Ryancare", a focal point of their electioneering. The plan receives only lukewarm support when survey respondents are given Republican talking points. The plan hurts Romney when respondents are given Democratic talking points.

Ryancare's downside seems to far outweigh Ryan's own potential upside. Can the Romney campaign really have decided that it would prefer to talk about Medicare, instead of Romney's record at Bain Capital? But the scrutiny of Ryancare will be yet another distraction from talking about the economy, which is an argument far easier won amid flagging consumer confidence.

3. There are other candidates who could have done more for Romney

If Mitt Romney had wanted someone who could truly make an impact in a swing state, then, Rob Portman in Ohio, or Bob McDonnell in Virginia, would have been far more attractive.

If he'd wanted a bona fide policy wonk, Bobby Jindal is an intellectual powerhouse who ran Louisiana's Department of Health and Hospitals at age 25.

If he'd wanted a candidate who wasn't another boring white guy, then Jindal, again, or Florida's Marco Rubio, would have fit the bill.

If he'd wanted a VP with blue-collar cred, then Tim Pawlenty is about as "Walmart-shopper" as they come.

Or if he'd wanted to go with an outsider? Jindal again, or New Jersey's Chris Christie.

Or what about a working mother? He could have chosen Kelly Ayotte, a conservative from the swing state of New Hampshire who was a tough-as-nails attorney general before becoming a US senator.

Conclusion

I may be missing something, but in electoral terms, Paul Ryan for VP makes little sense to me. There are other candidates who do what he does better – and Ryan has liabilities they don't.

It is worth noting also that vice-presidential picks rarely make much difference in general elections. But my judgment is that if Paul Ryan's selection does have any effect on this election, then it won't be in the direction Romney had wished.